Edit: It looks like the steps on the "installation Android" tab of http://docs.giderosmobile.com/interface/ads are obsolete. Instead, after adding the ads plugin to Gideros Studio, right click on it and select the framework you want to enable and enter your key. This is a really nice improvement!

After doing this for AppLovin, I'm able to play a video in my game using applovin:showAd("interstitial") (as shown on the "Methods" tab of the docs page).